Japanese Patent Application Publication Kokoku No 57-51551 discloses a pump of the type as will be called solenoid-operated reciprocating pump hereinafter in which a piston having a piston head slidably disposed in a cylinder is biased in one direction by a spring and is periodically attracted in the other opposite direction by means of an electromagnet including a plurality of magnetic poles located around the outer periphery of the piston to draw in and discharge fluid.
The piston of the solenoid-operated reciprocating pump as disclosed in said Patent Application Publication is supported for sliding movements axially of the cylinder by disposing the piston head within the cylinder and inserting the end of the piston opposite from the piston head in a tubular sleeve similar to the cylinder.
However, in such arrangement in which the piston is supported by the peripheries thereof at its fore and rear ends, that is, the periphery of the piston head and the periphery at the end of the piston opposite from the piston head, an increase in the diameter of the piston would necessarily require that both the periphery of the piston head and the periphery at the end of the piston opposite from the piston head be increased in diameter, resulting in an increase in the frictional drag of the piston, which will in turn reduce the discharging or suctioning efficiency of the solenoid-operated reciprocating pump.
In an attempt to overcome such problem, Japanese Utility Model Application Publication Kokoku No. 62-18712 discloses a solenoid-operated reciprocating pump in which a cantilever shaft is mounted in the pump casing and a piston having a bore the inner diameter of which is substantially equal to the outer diameter of said shaft is supported by inserting the shaft in said bore of the piston.
In said Utility Model the piston-returning spring is in the form of a coil spring and disposed between and secured to the piston and the casing where the cantilever shaft is fixed (see FIG. 1 of the Publication).
In this arrangement in which the piston has a bore formed extending therethrough and is supported by inserting the shaft in said bore, an increase in diameter of the piston would not necessarily require increasing the diameter of the bore correspondingly, not resulting in an excessive increase in the frictional drag of the piston.
The construction of the pump of the aforesaid Utility Model Application Publication Kokoku No. 62-18712 has, however, the disadvantage that as the piston is attracted by the electromagnet, the armature of the piston can be drawn farther toward those on either one side of a plurality of magnetic poles disposed around the piston than those on the other side, causing a circumferentially uneven wear on the piston, thereby reducing the useful life of the piston.
In order to resolve this disadvantage, Japanese Utility Model Application Publication Kokoku No. 55-45094 proposes a solenoid-operated reciprocating pump where a coil spring is rotatably supported at its one end against the pump casing by means of a spring retainer seated on a spherical ball while the spring is fixed at the other end to a piston, whereby the spring causes the piston to rotate gradually by torsional forces generated as the spring returns to its relaxed state from its compressed position to thereby prevent the piston from suffering an uneven wear.
Nevertheless, the coil spring is supported on the spherical surface, that is, in a one-point contact, so that the coil spring tends to lean from the central axis toward one or other side and flexed to a slight extent, which will in turn produce an undesirable biasing force toward one or other side of the cylinder on the piston.